foo := a.o b.o l.a c.o
one := $(patsubst %.o,%.c,$(foo))
# This is a shorthand for the above
two := $(foo:%.o=%.c)
# This is the suffix-only shorthand, and is alse equivalent to the above
three :=  $(foo:.o=.c)

all: 
	@echo $(one)
	@echo $(two)
	@echo $(three)